Nonaqueous electrolyte secondary battery

ABSTRACT

A nonaqueous electrolyte secondary battery wherein a material capable of occludeing/discharging lithium is used as a negative electrode material and a lithium-trasition metal composite oxide which contains Ni and Mn as the transition metal and has a layered structure is used as a positive electrode material is characterized in that a lithium-transition metal composite oxide having a BET specific surface area less than 3 m 2 /g and a pH of 11.0 or less when 5 g of the lithium-transition metal composite oxide is immersed in 50 ml of purified water is used as the positive electrode active material.

TECHNICAL FIELD

The present invention relates to a nonaqueous electrolyte secondarybattery and particularly to a nonaqueous electrolyte secondary batteryusing a lithium transition metal complex oxide containing Ni and Mn asthe positive electrode material.

BACKGROUND ART

In recent years, nonaqueous electrolyte batteries using carbon material,metallic lithium or lithium-alloying material for the negative activematerial and a lithium transition metal complex oxide represented byLiMO₂ (M indicates a transition metal) for the positive active materialhave been noted as high-energy-density secondary batteries.

A representing example of the lithium transition metal complex oxide isa lithium cobalt complex oxide (lithium cobaltate: LiCoO₂). This complexoxide has been already put to practical use as the positive activematerial of nonaqueous electrolyte secondary batteries.

The lithium transition metal complex oxide containing Ni or Mn as thetransition metal has been also studied for utility as the positiveactive material. For example, such compounds as containing all of thosethree transition metals Co, Ni, and Mn have been also studiedintensively (see, for example, Japanese Patent Registration Nos.2,561,556 and 3,244,314, and Journal of Powder Sources, 90(2000), pp176-181).

It is reported that, among those lithium transition metal complex oxidescontaining Co, Ni and Mn, a compound containing the same percentagecomposition of Ni and Mn, as represented by the formulaLiMn_(x)Ni_(x)Co_((1-2x))O₂, shows characteristically high thermalstability even in the charged state (high oxidation state)(Electrochemical and Solid-State Letters, 4(12), A200-A203(2001)).

It is also reported that the aforementioned complex oxide containingsubstantially the same percentage composition of Ni and Mn has a voltageof around 4 V, comparable to a voltage of LiCoO₂, and shows a highcapacity and a good charge-discharge efficiency (Japanese PatentLaying-Open No. 2002-42813). Thus, batteries using such a lithiumtransition metal complex oxide (such as represented by the formulaLiaMn_(b)Ni_(b)Co_((1-2b))O₂ (0≦a≦1.2 and 0<b≦0.5)) containing Co, Niand Mn and having a layered structure as the positive electrode materialare expected to exhibit high thermal stability even in the charged stateand accordingly exhibit markedly improved battery reliability.

The inventors of this application studied performance characteristics ofthe secondary lithium battery using the above-described lithiumtransition metal complex oxide as the positive active material. As aresult, they have found that when the battery is stored in the chargedstate at a temperature that is higher than 80° C., which temperature isestimated as the use condition of a portable phone inside an actual car,an increasing gas evolution due probably to a reaction between thepositive electrode and the electrolyte solution occurs to expand orswell the battery if having a configuration suitable for insertion inthe portable phone or the like. In an exemplary case where a batterycasing is made of a thin aluminum alloy or a thin aluminum laminatefilm, the battery when stored has been found to experience significantswelling and show marked deterioration such as capacity loss.

Batteries often use an outer casing formed of a thin aluminum alloy or athin aluminum laminate film to reduce their weights. As a solution tosuppress swelling of such batteries due to gas evolution duringhigh-temperature storage, a method is proposed wherein γ-butyrolactoneis contained as a solvent for an electrolyte solution in the amount of50-95% by volume (see, for example, Japanese Patent Laying-Open No.2000-235868). However, in this case, because γ-butyrolactone issusceptible to decomposition at a reducing side (at a negative electrodeside), the performance characteristics of the battery have beeninsufficient in total.

In Japanese Patent Laying-Open No. 2002-203552 and the 43rd BatterySymposium in Japan Meeting Abstracts, pp. 122-123, the use of a complexmetal oxide comprised mainly of Li and Ni and having a pH value of10.0-11.5 as the positive electrode material for a nonaqueouselectrolyte secondary battery is proposed to suppress swelling of thebattery during high-temperature storage. However, as a result of thedetailed study on lithium transition metal complex oxides containing Ni,Mn and Co and having a layered structure, the inventors of thisapplication have found that the use of such lithium transition metalcomplex oxides, even if kept within the specified pH range, results insignificant swelling of the batteries during high-temperature storage inthe charged state and thus the failure to obtain a sufficientimprovement.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a nonaqueouselectrolyte secondary battery using a lithium transition metal complexoxide containing Ni and Mn as the transition metal and having a layeredstructure, which can suppress the gas evolution during high-temperaturestorage in the charged state and thus exhibits improved high-temperaturestorage characteristics.

The nonaqueous electrolyte secondary battery of the present inventionuses a material capable of storing and releasing lithium as the negativeelectrode material and a lithium transition metal complex oxidecontaining Ni and Mn as the transition metal and having a layeredstructure as the positive electrode material. Characteristically, thelithium transition metal complex oxide has a BET specific surface areaof less than 3 m²/g and gives a pH value of not greater than 11.0 whenit is immersed in purified water in the amount of 5 g per 50 ml of thepurified water.

The gas evolution within the battery while stored in the charged stateat elevated temperatures can be suppressed and the high-temperaturestorage characteristics of the battery can be improved if the lithiumtransition metal complex oxide has a BET specific surface area of lessthan 3 m²/g and gives a pH value of not greater than 11.0 when measuredunder the above conditions in accordance with the present invention.Hence, swelling of a battery while stored in the charged state atelevated temperatures can be prevented, even if the battery is a sealed,nonaqueous electrolyte secondary battery using an outer casingsusceptible to deformation in case an internal pressure of the batterybuilds up.

In the present invention, the BET specific surface area of the lithiumtransition metal complex oxide is more preferably not greater than 2m²/g. When the BET specific surface area is kept within this range, gasevolution is further suppressed within the battery while stored in thecharged state at elevated temperatures and its high-temperature storagecharacteristics are further improved. Although not particularlyspecified, it is generally preferred that a lower limit of the BETspecific surface area does not fall below 0.1 m²/g.

In the present invention, the pH value of the lithium transition metalcomplex oxide refers to a pH value of a dispersion containing 5 g of thelithium transition metal complex oxide in 50 ml purified water, asdescribed above. In the present invention, the pH value is specified asbeing not greater than 11.0. A lower limit of the pH value is preferablyat least 9. Hence, the preferred pH value in the present invention is9.0-11.0. In Japanese Patent Laying-Open No. 2002-203552, the positiveactive material is specified as having a pH value of 10-11.5, asdescribed above. In the specification of Japanese Patent Laying-Open No.2002-203552, at paragraph No. 0028, a procedure utilized to measure thepH value is described. 2 g of the positive active material is dispersedin 100 ml purified water. After about 10 minutes of stirring, the pHvalue is measured for the resulting filtrate. On the other hand, in thepresent invention, 5 g of the positive active material is introduced in50 ml purified water. The resultant is ultrasonically treated for 10minutes and then filtered to obtain a filtrate. The pH value is measuredfor the filtrate, using a glass electrode, “Model D-21” of HORIBA, Ltd.

As such, Japanese Patent Laying-Open No. 2002-203552 and the presentinvention utilize different procedures in the measurement of the pHvalue. These different measurement procedures give different pH values,as will be described hereinafter.

Japanese Patent Laying-Open No. 2002-203552 describes that an alkalicontent, such as lithium carbonate, which remains on a surface of thepositive active material after synthesized, reacts with an electrolytesolution to generate a carbon dioxide or hydrocarbon gas that swells abattery. This reference also describes that generation of such a gasduring high-temperature storage can be suppressed by reducing a residuallithium carbonate content, i.e., adjusting the pH value of the activematerial to 11.5 or below. Also, in the 43rd Battery Symposium in JapanMeeting Abstracts, pp. 122-123, which discloses the same technicalsubject as in Japanese Patent Laying-Open No. 2002-203552, batteryswelling is described to occur as carbon dioxide absorbed in theremaining alkali content is released during high-temperature storage.

The positive active material used in Japanese Patent Laying-Open No.2002-203552 specifically contains Ni, Co and Al as the transition metal.As a result of detailed study on lithium transition metal complex oxidescontaining Ni, Mn and Co as the transition metal, the inventors of thisapplication and the others have found out the following:

(1) Batteries in some cases swell during high-temperature storage in thecharged state to experience marked deterioration of performancecharacteristics, even if the pH of the positive active material is keptwithin the range specified by Japanese Patent Laying-Open No.2002-203552.

(2) For batteries even if using the material having a pH of higher than11.5, i.e., having a higher residual alkali content, little swelling isobserved after high-temperature storage and no substantial deteriorationis observed during storage, if the batteries are stored in thedischarged state.

(3) The BET specific surface area of the positive active material insome cases affects the extent of swelling after high-temperature storagein the charged state and the degree of deterioration during storage ofthe battery, even if the pH of the positive active material is keptwithin the specified range.

From the foregoing, it appears that the nonaqueous electrolyte secondarybattery using a lithium transition metal complex oxide containing Ni, Mnand Co and having a layered structure experiences swelling duringhigh-temperature storage in the charged state due to a gas generated bya reaction between the positive active material and the electrolytesolution, as contrary to the description of Japanese Patent Laying-OpenNo. 2002-203552 which attributes the battery swelling to a gas generatedby a reaction of the residual alkali content with the electrolytesolution or to a gas released from the residual alkali content. Althougha detail is unknown, the residual alkali content is considered to act topromote the reaction between the electrolyte solution and the positiveactive material in the charged state.

In view of the above, the present invention specifies the positiveactive material as having a BET specific surface area of less than 3m²/g and a pH value of not greater than 11.0. This is believed to reducea reaction area between the positive active material and the electrolytesolution and also reduce the residual alkali content regarded as apromoter of a decomposition reaction, so that the reaction between theelectrolyte solution and the positive active material in the chargedstate is suppressed.

As described hereinabove, the present invention and Japanese PatentLaying-Open No. 2002-203552 utilize different procedures for determiningpH values. It appears that the hydroxide ion concentration in a filtratewhen determined using the procedure of this invention is five times ashigh as when determined using the procedure of Japanese PatentLaying-open No. 2002-203552 is utilized. This has been confirmed by thebelow-described Reference Experiment wherein the pH value whendetermined using the procedure of this invention is about 0.7 higherthan when determined using the procedure of Japanese Patent Laying-OpenNo. 2002-203552, which difference is almost as estimated.

The pH value of the positive active material can be adjusted to 11.0 orbelow by such methods as (1) rinsing the positive active material aftersynthesized, (2) reducing a ratio of Li to a transition metal in acharge stock before it is fired to provide the positive active material,and (3) reducing the amount of an unreacted Li compound by varying thefiring condition such as a firing temperature or firing time.

In the present invention, the BET specific surface area of the lithiumtransition metal complex oxide is less than 3 m²/g, more preferablywithin 2 m²/g, further preferably within 1.2 m²/g. As described above,the reduction of the BET specific surface area leads to a smallerreaction area between the positive active material and the electrolytesolution so that a reaction between the positive active material in thecharged state and the electrolyte solution can be retarded.

The lithium transition metal complex oxide for use in the presentinvention may preferably be represented by the formulaLi_(a)Mn_(x)Ni_(y)Co_(z)O₂ (wherein a, x, y and z are numbers satisfying0≦a≦1.2, x+y+z=1, x>0, y>0 and z≧0), for example. More preferably, thecomplex oxide contains substantially the same amount of nickel andmanganese. That is, x and y in the above formula have substantially thesame value. In the lithium transition metal complex oxide, nickel in itsnature has a high capacity but exhibits a low thermal stability duringcharge while manganese in its nature has a low capacity but exhibits ahigh thermal stability during charge. Accordingly, nickel and manganeseare preferably contained substantially in the same amount in order tobest balance their advantageous natures.

It is more preferred that x, y and z in the above formula satisfy0.25≦x≦0.5, 0.25≦y≦0.5 and 0≦z≦0.5.

When the positive and negative electrodes both have a rectangularelectrode surface and the nonaqueous secondary battery has a rectangularshape, a gas generated during storage tends to stay longer particularlybetween the electrodes. The present invention accordingly finds aparticular utility in nonaqueous secondary batteries which have arectangular shape and accommodate positive and negative electrodes bothhaving a rectangular electrode surface.

The positive and negative electrodes having a rectangular electrodesurface can be provided in various ways. For example, the positiveelectrode, the negative electrode and a separator interposedtherebetween may be rolled up and then compressed into a flat shape, orfolded such that their electrode surfaces have a rectangular shape.Rectangular positive and negative electrodes may be stacked with aseparator between them.

Swelling of a battery during storage occurs due to the buildup of abattery's internal pressure that results from the gas generation duringstorage. The present invention thus finds a particular utility insealed, nonaqueous secondary batteries using an outer casing susceptibleto deformation in case of internal pressure buildup.

An outer casing composed at least partly of an aluminum alloy oraluminum laminate film having a thickness of up to 0.5 mm is susceptibleto deformation in case of internal pressure buildup, for example. In thepresent invention, the aluminum laminate film refers to a laminate filmhaving an aluminum foil laminated at both sides with a plastic film. Theplastic film is generally made of polypropylene or polyethylene. Anouter casing composed at least partly of an iron alloy having athickness of up to 0.3 mm is also susceptible to deformation in case ofinternal pressure buildup. As the battery's internal pressure increases,such outer casings undergo deformation in a manner to swell at portionscomposed of those materials.

The negative electrode material for use in the present invention iscapable of storing and releasing lithium. Any material can be used, solong as it is generally useful for the negative electrodes of anonaqueous electrolyte secondary battery. Examples of useful negativeelectrode materials include graphite materials, metallic lithium andLi-alloying materials. Examples of Li-alloying materials includesilicon, zinc, germanium and aluminum.

The electrolyte for use in the nonaqueous electrolyte secondary batteryof the present invention can be selected, without limitation, from thoseknown to be useful for nonaqueous electrolyte secondary batteries suchas secondary lithium batteries. The type of the electrolyte solvent isnot particularly specified and can be illustrated by a mixed solvent ofcyclic carbonate and chain carbonate. Examples of cyclic carbonatesinclude ethylene carbonate, propylene carbonate, butylene carbonate andvinylene carbonate. Examples of chain carbonates include dimethylcarbonate, ethyl methyl carbonate and diethyl carbonate. The electrolytesolvent can also be illustrated by a mixed solvent of the aforementionedcyclic carbonate and an ether solvent such as 1,2-dimethoxyethane or1,2-diethoxyethane. Preferably, the ratio by volume of the cycliccarbonate to the chain carbonate or ether solvent (cycliccarbonate/chain carbonate or ether solvent) is 10/90-70/30.

The type of the electrolyte solute is not particularly specified and canbe illustrated by LiPF₆, LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂,LiN(CF₃SO₂)(C₄F₉SO₂), LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃, LiAsF₆, LiClO₄,Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂ and mixtures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view, showing the secondary lithium battery fabricatedin Examples in accordance with the present invention;

FIG. 2 is a photograph, showing the condition of the negative electrode(top surface) of the battery of Example 1 in accordance with the presentinvention, when charged after the storage test;

FIG. 3 is a photograph, showing the condition of the negative electrode(back surface) of the battery of Example 1 in accordance with thepresent invention, when charged after the storage test;

FIG. 4 is a photograph, showing the condition of the negative electrode(top surface) of the battery of Comparative Example 1 when charged afterthe storage test;

FIG. 5 is a photograph, showing the condition of the negative electrode(back surface) of the battery of Comparative Example 1 when chargedafter the storage test;

FIG. 6 is a photograph, showing the condition of the battery ofComparative Example 1 before the storage test;

FIG. 7 is a photograph, showing the condition of the battery ofComparative Example 1 after the storage test;

FIG. 8 is a schematic sectional view, showing a three-electrode beakercell;

FIG. 9 is a chart showing an XRD pattern of the positive electrode ofthe battery of Comparative Example 1 before the storage test; and

FIG. 10 is a chart showing an XRD pattern of the positive electrode ofthe battery of Comparative Example 1 after the storage test.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is below described in more detail by way ofExamples. It will be recognized that the following examples merelyillustrate the practice of the present invention but are not intended tobe limiting thereof. Suitable changes and modifications can be effectedwithout departing from the scope of the present invention.

EXAMPLE 1

(Preparation of Positive Active Material)

Li₂CO₃ and a coprecipitated hydroxide represented byMn_(0.33)Ni_(0.33)CO_(0.34)(OH)₂ were mixed in an Ishikawa automatedmortar such that a ratio in mole of Li to a group of the transitionmetals Mn, Ni and Co was brought to 1:1, heat treated in the ambientatmosphere at 950′ for 20 hours and then pulverized to obtain a lithiumtransition metal complex oxide having a mean secondary particle diameterof about 5 μm and represented by LiMn_(0.33)Ni_(0.33)CO_(0.34)O₂.

The obtained lithium transition metal complex oxide was rinsed withrunning water for 24 hours and then heat dried to provide a positiveactive material. The resulting positive active material had a BETspecific surface area of 1.2 m²/g.

(PH Value Measurement)

The obtained lithium transition metal complex oxide, weighing 5 g, wasintroduced in a 100 ml beaker to which 50 ml of purified water wassubsequently added. The beaker contents were ultrasonically treated for10 minutes and then filtered to collect a filtrate. A pH value of thefiltrate was measured using a glass electrode pH sensor (product ofHORIBA, Ltd., model D-21). Measurement was performed twice and anaverage value was recorded as a pH value. The pH value was 10.66.

(Fabrication of Positive Electrode)

The above-prepared positive active material, carbon as an electricalconductor and polyvinylidene fluoride as a binder were mixed in theratio (active material:conductor:binder) by weight of 90:5:5 and thenadded to N-methyl-2-pyrrolidone as a dispersing medium which wassubsequently kneaded to prepare a cathode mix slurry. The preparedslurry was coated on an aluminum foil as a current collector, dried andthen rolled by a pressure roll. Subsequent attachment of a currentcollecting tab completed fabrication of a positive electrode.

(Fabrication of Negative Electrode)

Artificial graphite as a negative active material and astyrene-butadiene rubber as a binder were mixed in an aqueous solutionof carboxymethylcellulose as a thickener so that the mixture containedthe active material, binder and thickener in the ratio by weight of95:3:2. The mixture was then kneaded to prepare an anode mix slurry. Theprepared slurry was applied onto a copper foil as a current collector,dried and rolled by a pressure roll. Subsequent attachment of a currentcollecting tab completed fabrication of a negative electrode.

(Preparation of Electrolyte Solution)

1 mole/liter of LiPF₆ was dissolved in a mixed solvent containingethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a 3:7 ratioby volume to prepare an electrolyte solution.

(Construction of Battery)

The above-fabricate positive and negative electrodes were stacked in apile with a separator between them, wound and then pressed into a flatshape to provide an electrode group. In a glove box maintained under anargon atmosphere, this electrode group was inserted into an inner spaceof an outer casing made of a 0.11 mm thick aluminum laminate. Afterintroduction of the electrolyte solution, the outer casing was sealed toencapsulate them.

FIG. 1 is a plan view, showing the secondary lithium battery Alconstructed in the manner as described above. In this secondary lithiumbattery, the aluminum laminate outer casing 1 is heat sealed at itsperiphery to form a sealed portion 2 for encapsulation. A positivecurrent collecting tab 3 and a negative current collecting tab 4 extendfrom an upper portion of the outer casing 1. Standard dimensions of thebattery were 3.6 mm in thickness, 3.5 cm in width and 6.2 cm in length.The constructed battery measured an initial thickness of 3.55 mm.

EXAMPLE 2

The procedure of Example 1 was followed, except that the firingtemperature in the preparation of the positive active material waschanged to 850° C., to construct a nonaqueous secondary battery A2. Theresulting active material had a BET specific surface area of 2.0 m²/gand a pH value of 10.84.

EXAMPLE 3

The procedure of Example 1 was followed, except that LiOH was used asthe Li source, the firing temperature was changed to 1,000′ and thefiring time was changed to 30 hours, to construct a nonaqueous secondarybattery A3. The resulting active material had a BET specific surfacearea of 0.40 m²/g and a pH value of 10.61.

COMPARATIVE EXAMPLE 1

The procedure of Example 1 was followed, except that LiOH was used asthe Li source, the firing temperature was changed to 1,000° C. and therinsing treatment was excluded, to construct a nonaqueous secondarybattery B1. The resulting active material had a BET specific surfacearea of 0.60 m²/g and a pH value of 11.12.

COMPARATIVE EXAMPLE 2

The procedure of Example 1 was followed, except that the ratio in moleof Li to the group of transition metals was changed to 1.15, firing wasperformed at 1,000′ for 30 hours and the rinsing treatment was excluded,to construct a nonaqueous secondary battery B2. The resulting activematerial had a BET specific surface area of 0.20 m²/g and a pH value of11.64.

(High-Temperature Storage Characteristics Evaluation)

Each of the above-constructed batteries A1, A2, A3, B1 and B2 wascharged at room temperature at a constant current of 650 mA to a voltageof 4.2 V, further charged at a constant voltage of 4.2 V to a currentvalue of 32 mA and then discharged at a constant current of 650 mA to avoltage of 2.75 V to measure a discharge capacity (mAh) of the batterybefore storage.

Next, each battery was charged at room temperature at a constant currentof 650 mA to a voltage of 4.2 V, further charged at a constant voltageof 4.2 V to a current value of 32 mA and then stored in a thermostaticchamber at 85° C. for 3 hours. After storage, the battery was cooled atroom temperature for 1 hour and then measured for thickness. A thicknessincrease (mm) was calculated from comparison between the measuredthickness and the initial thickness of the battery and was evaluated asswelling of the battery after high-temperature storage.

The pH values and BET specific surface areas of respective positiveactive materials of the batteries, as well as the swelling evaluationresults of the batteries after storage, are listed in Table 1. Theswelling rate for each battery is expressed as thicknessincrease/initial battery thickness×100. TABLE 1 pH Value of BET Value ofBattery Swelling Positive Positive Active After Swelling Rate ActiveMaterial High-Temperature of Battery Battery Material (m²/g) Storage(mm) (%) Ex. 1 A1 10.66 1.2 0.52 15 Ex. 2 A2 10.84 2.0 0.94 26 Ex. 3 A310.61 0.40 0.43 12 Comp. B1 11.12 0.60 2.85 75 Ex. 1 Comp. B2 11.64 0.202.56 69 Ex. 2

As can be appreciated from the results shown in Table 1, the batteriesA1-A3 of Examples 1-3 in accordance with the present invention, eachusing the positive active material having a pH value of not greater than11.0 and a BET specific surface area of less than 3 m²/g, experiencelittle swelling during storage and thus show small swelling rates afterstorage.

Next, each battery after storage was discharged at room temperature at aconstant current of 650 mA to a voltage of 2.75 V to measure theremaining capacity (mAh). The value obtained by dividing the remainingcapacity by the discharge capacity of the battery before storage wasrecorded as a retention rate.

After measurement of the remaining capacity, the battery was charged ata constant current of 650 mA to a voltage of 4.2 V, further charged at aconstant voltage of 4.2 V to a current value of 32 mA and thendischarged at a constant current of 650 mA to a voltage of 2.75 V tomeasure a restored capacity. The value obtained by dividing the restoredcapacity by the discharge capacity of the battery before storage wasrecorded as a restoration rate.

The thus measured discharge capacity before storage, remaining capacity,retention rate, restored capacity and restoration rate for each batteryare shown in Table 2. TABLE 2 BET Value of Discharge Remaining RestoredPositive Capacity Capacity Capacity Active Before (mAh) (mAh) MaterialStorage (Retention (Restoration Battery (m²/g) (mAh) Rate) Rate) Ex. 1A1 1.2 643.2 561.3 578.5 (87.3%) (89.9%) Ex. 2 A2 2.0 630.1 538.0 551.2(85.4%) (87.5%) Ex. 3 A3 0.40 600.6 510.9 528.6 (85.1%) (88.0%) Comp. B10.60 673.0 483.8 506.3 Ex. 1 (71.9%) (75.2%) Comp. B2 0.20 682.1 526.2551.2 Ex. 2 (77.1%) (80.8%)

As can be clearly seen from Table 2, the remaining capacity afterhigh-temperature storage and the restoration capacity are both high inthe batteries A1-A3 of Examples 1-3 in accordance with the presentinvention. This demonstrates that the batteries in accordance with thepresent invention show improved high-temperature storagecharacteristics.

As can also be appreciated from Tables 1 and 2, the batteries ofExamples 1 and 3, each using the positive active material having a BETspecific surface area of not greater than 1.2 m²/g, show particularlyreduced swelling and deterioration after storage. This suggests that theBET specific surface area is more preferably not greater than 1.2 m²/g.

(Observation of Negative Electrode Condition After Storage Test)

The condition of the negative electrode after the storage test wasobserved for the battery A1 of Example 1 and the battery B1 ofComparative Example 1. Specifically, each battery after the storage testwas charged at a constant current of 650 mA to a voltage of 4.2 V andfurther charged at a constant voltage of 4.2 V to a current value of 32mA and then disassembled to remove its negative electrode forobservation. FIGS. 2 and 3 both show the negative electrode ofExample 1. FIG. 2 shows its top surface and FIG. 3 shows its backsurface. FIGS. 4 and 5 both show the negative electrode of ComparativeExample 1. FIG. 4 shows its top surface and FIG. 5 shows its backsurface.

As apparent from comparison between FIGS. 2-5, in the battery ofComparative Example 1 which showed marked swelling after the storagetest, a wide distribution of unreacted dark portions is observed in aportion charged and discolored to gold (appearing white in thedrawings). The formation of these unreacted dark portions is believeddue to a gas which is generated during storage and stays between theelectrodes in the form of bubbles that hinder a reaction at electrodeportions in contact therewith.

On the other hand, in the battery of Example 1 in accordance with thepresent invention, such unreacted portions are not observed in thecharged negative electrode. This demonstrates the uniform occurrence ofa charge-discharge reaction.

It is evident from the above that the use of the lithium transitionmetal complex oxide having the BET specific surface area and the pHvalue within the respective ranges specified in the present inventionreduces gas generation during storage, permits uniform occurrence of acharge-discharge reaction and suppresses deterioration of batterycharacteristics after high-temperature storage.

FIG. 6 is a photograph which shows the battery of Comparative Example 1before the storage test and FIG. 7 is a photograph which shows thebattery of Comparative Example 1 after the storage test. As apparentfrom comparison between FIGS. 6 and 7, the storage test causes swellingof the outer casing of the battery.

(Reference Experiment 1)

In this Experiment, a secondary lithium battery was constructed by usingan aluminum alloy can made of a 0.5 mm thick aluminum alloy sheet(Al—Mn—Mg alloy, JIS A 3005, proof stress of 14.8 kgf/mm²) as an outercasing. The occurrence of swelling was confirmed after the storage testfor the secondary lithium battery in case of using such an outer casingand the positive active material of Comparative Example 1.

(Construction of Reference Battery 1)

The procedure of Example 1 was followed, except that the above-describedaluminum alloy can was used as the outer casing, LiCoO₂ was used as thepositive active material and the standard battery dimensions werechanged to 6.5 mm in thickness, 3.4 cm in width and 5.0 cm in length, toconstruct a secondary lithium battery Y1. The constructed battery had aninitial thickness of 6.01 mm.

(Construction of Reference Battery 2)

The procedure of Example 1 was followed, except that the above-describedaluminum alloy can was used as the outer casing, the positive activematerial of Comparative Example 1 was used and the standard batterydimensions were changed to 6.5 mm in thickness, 3.4 cm in width and 5.0cm in length, to construct a secondary lithium battery Y2. Theconstructed battery had an initial thickness of 6.04 mm.

(Evaluation of Battery Swelling After High-Temperature Storage)

Each of the above-constructed batteries was charged at room temperatureat a constant current of 950 mA to a voltage of 4.2 V, further chargedat a constant voltage of 4.2 V to a current value of 20 mA and thenstored in a thermostatic chamber at 85° C. for 3 hours. After storage,the battery was cooled at room temperature for 1 hour and then measuredfor thickness. The battery swelling was evaluated after high-temperaturestorage in the same manner as in Example 1. The evaluation results areshown in Table 3. TABLE 3 Battery Swelling After High-TemperatureStorage Battery (mm) Ref. Battery 1 Y1 0.25 (4.2%) Ref. Battery 2 Y21.42 (23.5%)

As can be clearly seen from Table 3, the battery Y2 using the positiveactive material of Comparative Example 1 shows a very large swelling of1.42 mm after high-temperature storage. This demonstrates that thebattery deforms due to an increasing internal pressure even when it usesthe 0.5 mm thick aluminum alloy can as its outer casing. It is thereforeexpected that when the present invention is applied to a battery usingsuch an outer casing, gas generation within the battery duringhigh-temperature storage can be retarded to result in a marked reductionof battery swelling.

(Reference Experiment 2)

In order to investigate a cause of deterioration of the battery ofComparative Example 1 in storage, the battery after the storage test wasdisassembled, its positive electrode was collected and subjected to thefollowing experiment.

(Characteristic Experiment of Electrode)

The three-electrode beaker cell shown in FIG. 8 was constructed usingthe above-collected positive electrode as a working electrode, a lithiummetal as counter and reference electrodes, and an electrolyte solutionprepared by dissolving 1 mole/liter of LiPF₆ in a mixed solvent(EC/EMC=3/7 (volume ratio)) of ethylene carbonate (EC) and ethyl methylcarbonate (EMC). As shown in FIG. 8, the working electrode 11, thecounter electrode 12 and reference electrode 13 are immersed in theelectrolyte solution 14.

The constructed cell was charged at a current density of 0.75 mA/cm² to4.3 V (vs. Li/Li⁺) and then discharged at a current density of 0.75mA/cm² to 2.75 V (vs. Li/Li⁺) to determine a capacity (mAh/g) per gramof the positive active material. Next, the constructed cell was chargedat a current density of 0.75 mA/cm² to 4.3 V (vs. Li/Li⁺) and thendischarged at a current density of 3.0 mA/cm² to 2.75V (vs. Li/Li⁺) todetermine a capacity (mAh/g) per gram of the positive active material.Also, an average electrode potential during discharge at a currentdensity of 0.75 mA/cm² was calculated from the following equation. Thesame testing was performed for the positive electrode before the storagetest to compare characteristics of the positive electrode before andafter the storage test.[Average electrode potential (V vs. Li/Li⁺)]=[gravimetric energy density(mWh/g) during discharge]÷[capacity (mAh/g) per gram of positive activematerial]

The results of the charge-discharge test at discharge currents of 0.75and 3.0 mA/cm² are shown in Tables 4 and 5, respectively. TABLE 4Positive Discharge Average Electrode Electrode of Capacity EnergyDensity Potential Comp. Ex. 1 (mAh/g) (mWh/g) (V vs. Li/Li⁺) BeforeStorage 158.3 602.8 3.807 Test After Storage 155.6 589.3 3.787 Test

TABLE 5 Ratio of Discharge Capacity Positive at 3.0 mA/cm² to DischargeElectrode of Discharge Capacity Capacity at 0.75 mA/cm² Comp. Ex. 1(mAh/g) (%) Before Storage 145.8 92.1 Test After Storage 143.5 92.2 Test

As apparent Tables 4 and 5, the difference in performance of thepositive electrode before and after storage was little appreciated. Thisappears to demonstrate that the positive electrode or its activematerial experiences no deterioration in high-temperature storage.

(Measurement of XRD Pattern Before and After Storage)

The preceding positive electrode (in the discharged state) collectedafter the storage test and the positive electrode before the storagetest were both subjected to x-ray diffraction measurement using Cu—Kα asan X-ray source. The measurement results are shown in FIGS. 9 and 10.FIG. 9 shows an XRD pattern for the positive electrode before thestorage test and FIG. 10 shows an XRD pattern for the positive electrodeafter the storage test. As can be appreciated from comparison betweenFIGS. 9 and 10, the change in XRD pattern of the positive electrodebefore and after the storage test is not significant. It is thereforebelieved that no structural change occurs in the positive activematerial before and after the storage test.

From the foregoing, it appears that deterioration of the battery duringstorage is unlikely due to the structural change of the positive activematerial or deterioration of the positive electrode but more likely dueto a gas generated during storage that stays between the electrodes tocause uneven occurrence of a charge-discharge reaction. In accordancewith the present invention, gas generation during storage can besuppressed. Therefore, deterioration of battery characteristics duringstorage can also be suppressed.

(Reference Experiment 3)

In this Experiment, the influence of the different pH measurementprocedures, i.e., those disclosed in the present invention and inJapanese Patent Laying-Open No. 2002-203552, on the measured pH valuewas investigated. The measurement procedures described in the presentinvention and Japanese Patent Laying-OpenNo. 2002-203552 were utilizedto measure respective pH values of the positive active materials ofExample 1 and Comparative Example 2. The measurement results are shownin Table 6.

According to the measurement procedure of the present invention, 5 g ofeach positive active material was introduced in 50 ml of purified water,the resultant was ultrasonically treated for 10 minutes and thenfiltered, and a pH value of the resulting filtrate was measured.According to the measurement procedure of Japanese Patent Laying-OpenNo. 2002-203552, 2 g of each positive active material was introduced in100 ml of purified water, the resultant was ultrasonically treated for10 minutes and then filtered, and a pH value of the resulting filtratewas measured. TABLE 6 pH Value Measured pH Value Measured According to aAccording to a Procedure of the Procedure of Patent pH Value PresentInvention Literature 5 Difference Ex. 1 10.66 9.96 0.72 Comp. 11.6410.92 0.70 Ex. 2

Compared with the measurement procedure of Japanese Patent Laying-OpenNo. 2002-203552, the measurement procedure of the present inventionintroduces a fivefold amount of the active material into a unit volumeof purified water. It is therefore expected that when the activematerial is immersed in water, an alkali concentration in themeasurement procedure of the present invention is five times as high asin the other measurement procedure. Accordingly, the pH value isestimated to increase by log 5, i.e., about 0.70. The experiment resultsshown in Table 6 indicate pH values as such estimated. The pH value whenobtained by the measurement procedure of the present invention istherefore regarded as being about 0.70 higher than when obtained by themeasurement procedure of Japanese Patent Laying-Open No. 2002-203552.

(Reference Experiment 4)

The batteries of Comparative Examples 1 and 2 while both in thedischarged state (650 mA, end voltage of 2.75 V) were subjected to thesame storage test as described above to observe swelling thereof beforeand after storage and deterioration thereof in storage. Since they werestored in the discharged state, measurement of the remaining capacityafter storage was not carried out. After storage, each battery wascycled under the same charge-discharge conditions as in Example 1 tomeasure a restored capacity and a restoration rate. The measurementresults are shown in Table 7. TABLE 7 Battery Swelling DischargeRestored After High- Capacity Capacity (mAh) Temperature Before Storage(Restoration Battery Storage (mm) (mAh) Rate) Comp. B1 0.161 664.0 658.3Ex. 1 (99.1%) Comp. B2 0.096 695.6 690.9 Ex. 2 99.3%

As can be appreciated from comparison between Tables 1, 2 and 7, eventhe batteries of Comparative Examples 1 and 2 experience little swellingand deterioration in storage, if they are stored in the dischargedstate. This demonstrates that batteries using the positive activematerial of the present invention experience swelling and deteriorationonly when they are stored in the charged state. Therefore, theoccurrence of gas generation and battery swelling duringhigh-temperature storage is believed due to some factor other than areaction between a residual alkali content and an electrolyte solutionor gas release from the residual alkali content.

UTILITY IN INDUSTRY

The use of a lithium transition metal complex oxide having theparticular BET specific surface area and pH value as the positiveelectrode material, in accordance with the present invention, retardsgas generation within a battery during high-temperature storage in thecharged state, restrains swelling of the battery and suppressesdeterioration of battery characteristics in high-temperature storage.

1. In a nonaqueous electrolyte secondary battery using a materialcapable of storing and releasing lithium as a negative electrodematerial and a lithium transition metal complex oxide containing Ni andMn as the transition metal and having a layered structure as a positiveelectrode material, said secondary battery being characterized in thatsaid lithium transition metal complex oxide has a BET specific surfacearea of less than 3 m²/g and gives a pH value within the range of9.0-11.0 when it is immersed in purified water in the amount of 5 g per50 ml of the purified water, and that an outer casing of said battery iscomposed at least partly of an aluminum alloy or aluminum laminate filmhaving a thickness of up to 0.5 mm and susceptible to deformation incase of internal pressure buildup due to gas generation within thebattery during storage. 2-4. (canceled)
 5. The nonaqueous electrolytesecondary battery as recited in claim 1, characterized in that saidlithium transition metal complex oxide is represented by the formulaLi_(a)Mn_(x)Ni_(y)Co_(z)O₂ (wherein a, x, y and z are numbers satisfying0≦a≦1.2, x+y+z=1, x>0, y>0 and z≧0).
 6. The nonaqueous electrolytesecondary battery as recited in claim 1, characterized in that saidlithium transition metal complex oxide contains substantially the sameamount of nickel and manganese.
 7. The nonaqueous electrolyte secondarybattery as recited in claim 1, characterized in that said lithiumtransition metal complex oxide has a BET specific surface area of notgreater than 2 m²/g.
 8. The nonaqueous electrolyte secondary battery asrecited in claim 5, characterized in that said lithium transition metalcomplex oxide contains substantially the same amount of nickel andmanganese.
 9. The nonaqueous electrolyte secondary battery as recited inclaim 5, characterized in that said lithium transition metal complexoxide has a BET specific surface area of not greater than 2 m²/g. 10.The nonaqueous electrolyte secondary battery as recited in claim 6,characterized in that said lithium transition metal complex oxide has aBET specific surface area of not greater than 2 m²/g.
 11. The nonaqueouselectrolyte secondary battery as recited in claim 8, characterized inthat said lithium transition metal complex oxide has a BET specificsurface area of not greater than 2 m²/g.